One subject of the invention is novel catalytic systems for the production of hydrogen by hydrolysis of metal borohydrides and also devices for producing hydrogen that use these novel systems. More particularly, the invention relates to catalytic systems that promote the hydrolysis reaction of metal borohydrides to hydrogen.
Hydrogen is what is known as a “clean” fuel, since it reacts with oxygen in suitable devices to give electrical energy and water. It is used as fuel in certain combustion engines and in fuel cells. The use of hydrogen as a fuel is therefore advantageous from the environmental point of view. It is consequently desirable to provide methods of producing hydrogen which are satisfactory and which can be carried out in devices of reduced size or miniaturized devices so as to be able to integrate this production of hydrogen into microsystems.
The storage and production of hydrogen in situ constitute the main hindrance to the emergence of fuel cells. One of the most promising systems for storing and producing hydrogen uses compounds of metal borohydride type as a means for storing hydrogen. Sodium borohydride in particular is particularly promising since it can be easily dissolved in an aqueous medium.
Metal catalysts are known, especially from document U.S. Pat. No. 6,534,033, which make it possible to activate the reduction of metal hydrides, and in particular of metal borohydrides, to hydrogen, this reaction taking place in the presence of water, according to the scheme below:

This reaction is exothermic. The borate produced is non-toxic and can be regenerated to borohydride.
Besides sodium borohydride (NaBH4), other metal hydrides may be used to carry out this reaction.
The catalysts that can be used for carrying out this reaction and which are taught by U.S. Pat. No. 6,534,033 include, in particular, metals ranging from Group IB to Group VIIIB of the Periodic Table of the Elements, or compounds obtained from these metals. Mention may especially be made of ruthenium, copper, cobalt, iron, nickel, manganese, rhodium, rhenium, platinum, palladium, chromium, silver, osmium and iridium.
Various devices that make it possible to carry out this reaction have been proposed. Generally, a hydrogen generator functions according to the principle represented schematically in FIG. 1.
An aqueous solution of sodium borohydride (1.1) is injected from a reservoir (1) into a catalytic reactor (2) using a pump (3). The borohydride is hydrolysed in contact with the catalysts present in the reactor. The reaction products are conveyed to a separation chamber (4) in which the hydrogen is separated from the borate formed. This borate is then recovered in a reservoir (5) in order to be possibly recycled, whereas the hydrogen is conveyed to the fuel cell (6). The hydrogen demand is managed by controlling the flow rate of the aqueous metal borohydride solution.
The main drawback of this technology lies in the need to preheat the aqueous metal borohydride solution. This is because the kinetics of this reaction, in accordance with the Arrhenius law, are a function of the temperature. In order to promote a high reaction rate, it is therefore necessary to activate the system via a heat input, especially when the generator is used at low temperature.
Moreover, one limiting factor of the reaction is the solubility of the initial borohydrides. The solubility of sodium borohydride in water is only 35%, that of the reaction product, NaBO2, is only 22% at room temperature. It is therefore necessary to make provision either for using a very dilute solution of sodium borohydride (to avoid the precipitation of the product of the reaction), or for supplying water to the reaction as it progresses. But to maximize the energy density of the system it is sought to work with sodium borohydride concentrations that are as high as possible. The fact of heating the metal borohydride solution makes it possible to increase the solubility of the latter and above all that of the product of the reaction, and to work with more concentrated solutions.
One solution to this problem is proposed in the US 2005/0276746 patent application: the hydrogen generator comprises a system for heating the aqueous solution. The element that provides the heating may be external, it is then a heat source added to the conventional devices. But this solution has the drawback of adding to the space requirements of the device while it is sought to have hydrogen generators that are as compact as possible. Another solution consists in recovering the heat produced by the hydrolysis reaction of the metal borohydride in order to preheat the solution. In the latter case, the heating is not available at the start-up of the device.
Therefore, there remains the need for means that make it possible to carry out the conversion reaction of metal borohydrides to hydrogen at high reactant concentrations, in order to have high hydrogen yields with high reaction kinetics from the start-up of the device. And it was desired that this reaction can be carried out in simple reactors that are not very bulky, so as to favour the yield of hydrogen relative to the bulkiness of the device.
The solution to this problem lies in the development of novel catalytic systems which will be described below.